A traditional fluid-filled power transmitting device is equipped with a lock-up clutch to improve the power transmitting efficiency during operation of the device. For example, the lock-up clutch is fully engaged or disengaged, depending on an operating condition of the device. To control the lock-up clutch, a lock-up clutch controller has been employed. An example of such a controller is disclosed in U.S. Pat. No. 5,865,709 (hereafter “the '709 patent”), entitled “Apparatus for controlling vehicle lock-up clutch, wherein engine output is reduced upon releasing action of lock-up clutch.” The '709 patent is directed towards providing an apparatus to reduce a releasing shock of a motor vehicle when the lock-up clutch of the motor vehicle is switched from a fully engaged state to a fully released state.
However, a rough shift of power transmission or a shift with unacceptable acceleration can also be experienced in the fluid-filled power transmitting device when the lock-up clutch of the device is switched from a fully released state to a fully engaged state. During the engagement of the lock-up clutch, the torque transmitted from the engine to the transmission through the lock-up clutch causes rapid acceleration of a machine equipped with the device. Such a sudden acceleration can cause loss of precise machine control or degradation of driving comfortableness of the driver.